The field of the invention is magnetic resonance imaging (“MRI”) methods and systems. More particularly, the invention relates to delayed enhancement cardiac MRI.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically proven pulse sequences and they also enable the development of new pulse sequences.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space”. Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a roster scan-like pattern sometimes referred to as a “spin-warp”, a “Fourier”, a “rectilinear” or a “Cartesian” scan. The spin-warp scan technique is discussed in an article entitled “Spin-Warp MR Imaging and Applications to Human Whole-Body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (ΔGy) in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed.
There are many other k-space sampling patterns used by MRI systems These include “radial”, or “projection reconstruction” scans in which k-space is sampled as a set of radial sampling trajectories extending from the center of k-space as described, for example, in U.S. Pat. No. 6,954,067. The pulse sequences for a radial scan are characterized by the lack of a phase encoding gradient and the presence of a readout gradient that changes direction from one pulse sequence view to the next. There are also many k-space sampling methods that are closely related to the radial scan and that sample along a curved k-space sampling trajectory rather than the straight line radial trajectory. Such pulse sequences are described, for example, in “Fast Three Dimensional Sodium Imaging”, MRM, 37:706-715, 1997 by F. E. Boada, et al. and in “Rapid 3D PC-MRA Using Spiral Projection Imaging”, Proc. Intl. Soc. Magn. Reson. Med. 13 (2005) by K. V. Koladia et al and “Spiral Projection Imaging: a new fast 3D trajectory”, Proc. Intl. Soc. Magn. Reson. Med. 13 (2005) by J. G. Pipe and Koladia.
An image is reconstructed from the acquired k-space data by transforming the k-space data set to an image space data set. There are many different methods for performing this task and the method used is often determined by the technique used to acquire the k-space data. With a Cartesian grid of k-space data that results from a 2D or 3D spin-warp acquisition, for example, the most common reconstruction method used is an inverse Fourier transformation (“2DFT” or “3DFT”) along each of the 2 or 3 axes of the data set. With a radial k-space data set and its variations, the most common reconstruction method includes “regridding” the k-space samples to create a Cartesian grid of k-space samples and then perform a 2DFT or 3DFT on the regridded k-space data set. In the alternative, a radial k-space data set can also be transformed to Radon space by performing a 1 DFT of each radial projection view and then transforming the Radon space data set to image space by performing a filtered backprojection.
Because it requires time to acquire a complete k-space MR data set, subject motion presents a problem in many clinical applications. Motion due to respiration and cardiac motion can produce image artifacts such as blurring or ghosting. There are many strategies used to suppress such artifacts. These include cardiac or respiratory gating techniques that acquire MR data only during certain phases of the cardiac or respiratory cycle. In gated cardiac MRI, for example, one or more k-space views of the heart (“segment”) are acquired a preset time interval after the ECG triggered gating signal is produced. View segments for the image are acquired over a plurality of heart beats at the same preset time interval until sufficient data is acquired to reconstruct an image depicting the heart at that particular cardiac phase. By acquiring 8 to 16 views in each segment, a complete image can be acquired in one breath hold, thus eliminating respiratory motion issues. Typically, during a segmented cardiac MRI scan, segments of data will be acquired at a succession of cardiac phases during each cardiac cycle, or R-R interval, so that a plurality of images may be reconstructed at the conclusion of the scan which depict the heart at a corresponding succession of cardiac phases.
Delayed enhancement (DE) magnetic resonance imaging (MRI) is a cardiac MRI method for myocardial viability imaging. This method distinguishes healthy and infarcted myocardium. The identification of viable myocardium is useful for predicting which patients will have improved left ventricular (LV) ejection fractions and improved survival after revascularization. The transmural extent of infarcted tissue as determined by DE MRI has been shown to predict functional recovery post-revascularization procedures such as coronary bypass surgery.
DE MRI involves the injection of a bolus of a Gadolinium-based contrast agent called Gd-DTPA. Starting approximately ten minutes after the injection, Gd-DTPA preferentially pools in the areas of infarct due to differences in the wash-in times and distribution volumes between viable and non-viable tissue. The presence of a larger concentration of Gd-DTPA causes T1 shortening in infarcted tissue. The standard MRI pulse sequence for visualizing these infarcts is an inversion recovery gradient echo (IR-GRE) pulse sequence which takes advantage of the short T1 time of infarcted tissue to create images where viable tissue is nulled (dark) while infarcted tissue appears bright. A limitation of IR-GRE imaging for DE MRI is that blood in the left ventricle also appears bright. This makes it difficult to determine the border between blood and infarcted tissue and it can also result in the failure to detect small subendocardial infarcts that appear to be LV blood.
Cine imaging of the beating heart with MRI is performed during cardiac studies to visualize the wall thickness and systolic wall thickening throughout the cardiac cycle. With cine imaging complete k-space image data sets are acquired at a succession of cardiac phases so that the myocardium can be imaged throughout a complete cardiac cycle. Cine imaging is typically acquired with very short TR steady-state free precession (SSFP) imaging pulse sequences. These cine images are used to detect dysfunctional myocardium that may appear viable on DE MRI images. Cine imaging is also used to determine the LV ejection fraction to determine the overall pumping capacity of the heart.
DE MRI and cine images are acquired in a short axis view of the heart, and 10-15 slices are acquired during each scan to cover the entire left ventricle. Imaging a single anatomical slice for DE MRI or cine imaging requires a 10-20 second breath-hold. Currently, DE MRI and cine imaging are acquired separately, thus resulting in 20-30 of these breath holds. This can be quite difficult for some patients, and results in long scan times.
A real-time method for DE MRI imaging is disclosed by Guttman M A, Dick A J, Raman V K, et al. “Imaging of myocardial infarction for diagnosis and intervention using real-time interactive MRI without ECG-gating or breath-holding”, Magn Reson Med 2004; 42:354-61, to alleviate the requirements of breath-holding and cardiac gating. However, this real-time method has a lower spatial resolution than conventional DE MRI, and the temporal resolution of 2-6 frames per second is not adequate for analyzing wall motion, thus still necessitating a separate cardiac cine scan. A cardiac-gated cine delayed enhancement pulse sequence for acquiring viability and wall motion images simultaneously is disclosed in published U.S. Patent Application No. 20050245812 filed on Nov. 3, 2005 and entitled “Acquiring Contrast-Enhanced, T1-Weighted, Cine Magnetic Resonance Images”. This method uses a single-shot acquisition over approximately 300 ms instead of a segmented acquisition, and thus blurring is introduced into the images, particularly during systole. This method also maintains the same tissue contrast throughout the cine images.